1. Field of the Invention
The present invention relates generally to a solid immersion lens (SIL), a focusing lens using such solid immersion lens, an optical pickup apparatus, an optical (or magneto-optical) recording and reproducing apparatus and a method of forming a solid immersion lens. More particularly, the present invention relates to a solid immersion lens suitable for use with a so-called near field optical recording and reproducing system in which a numerical aperture of a focusing lens is increased by using a material in which an optical lens has a large refractive index to thereby record and reproduce information on and from an optical (or magneto-optical) recording medium, a focusing lens using such solid immersion lens, an optical pickup apparatus, an optical recording and reproducing apparatus and a method of forming a solid immersion lens.
2. Description of the Related Art
Optical recording mediums (including a magneto-optical recording medium), which are represented by a compact disc (CD), a mini disc MD) and a digital versatile disc (DVD), are widely used as storage mediums for storing therein music information, video information, data, programs and the like. However, as music information, video information, data, programs and the like are increasingly improved to become higher in tone quality, image quality, become longer in record time and play time and also become larger in storage capacity, it is desired that optical recording mediums should be increased in storage capacity and that optical recording and reproducing apparatus (including magneto-optical recording and reproducing apparatus) capable of recording and reproducing such mass-storage optical recording mediums should be realized.
Accordingly, in order to meet with the above-mentioned requirements, in the optical recording and reproducing apparatus, a wavelength of a semiconductor laser, for example, of its light source is shortened and a numerical aperture of a focusing lens is increased and thereby a diameter of a spot of light beam converged through the focusing lens is reduced.
For example, with respect to semiconductor lasers, a GaN semiconductor laser which emits laser light of which wavelength is reduced from 635 nm of related-art red laser to 400 nm band was put into practice and thereby a diameter of a spot of light beam can be reduced. Also, with respect to wavelength which can be reduced more than the above-mentioned short wavelength, for example, a far ultraviolet solid-state laser which can continuously emit laser light of a single wavelength of 266 nm is now commercially available on the market (manufactured by Sony Corporation under the trade name of UW-1020A) and hence a diameter of a spot of laser light can be reduced more. In addition, a twice wave laser (266 nm band) of an Nd:YAG laser, a diamond laser (235 nm band), a twice wave laser (202 nm band) of a GaN laser and so on are now under study and development.
A so-called near field optical recording and reproducing system has been studied in which a focusing lens with a numerical aperture greater than 1 can be realized by using an optical lens with a large numerical aperture represented by a solid immersion lens (SIL) and in which recording and reproducing can be carried out by making the objective surface of this focusing lens become close to an optical recording medium with a distance of approximately 10/1 of a wavelength of its light source (see Cited Patent Reference, U.S. Pat. No. 5,125,750, for example).
In this near field optical recording and reproducing system, it is important to maintain a distance between the optical recording medium and the focusing lens in the optical contact state with high accuracy. Also, since a diameter of a bundle of light introduced into the focusing lens from the light source is reduced and the distance between the optical recording medium and the focusing lens is reduced to become very small, which is less than approximately several 10 s of nanometers, an inclination margin between the optical recording medium and the focusing lens, that is, so-called can become very small and hence it is unavoidable that the focusing lens is largely restricted from a shape standpoint.
FIG. 1 of the accompanying drawings is a schematic diagram showing an arrangement of an example of a solid immersion lens (SIL). As shown in FIG. 1, a solid immersion lens 11 and an optical lens 12 can be located, in that order, from the objective side such as an optical recording medium 10 (including a magneto-optical recording medium), thereby constructing a near field focusing lens. As shown in FIG. 1, the solid immersion lens 11 is formed like a hemispherical shape with a radius of curvature r or a hyper-hemispherical lens (hyper-hemispherical shape in the example of FIG. 1). When the solid immersion lens 11 is shaped like the hemispherical solid immersion lens, the thickness thereof extending along the optical axis is selected to be r. When the solid immersion lens 11 is shaped like the hyper-hemispherical solid immersion lens as in the illustrated example of FIG. 1, the thickness thereof extending along the optical axis is selected to be r (1+1/n) where n represents the refractive index.
When the focusing lens having the above-mentioned arrangement is applied to an optical recording and reproducing apparatus, for example, it is mounted on an optical pickup apparatus having a biaxial actuator and thereby a distance between the optical recording medium and the focusing lens can be maintained in an optical contact state. When the above-mentioned focusing lens is applied to magneto-optical recording, a magnetic head apparatus for use in magnetic recording and reproduction is assembled into the optical pickup apparatus and thereby a distance between the optical recording medium and the focusing lens can be similarly maintained in an optical contact state.
In the above-mentioned near field optical recording and reproducing system, in order to stably control the focusing lens which is driven in the focusing direction and/or tracking direction relative to the optical recording medium and also in order to stably record and reproduce the optical recording medium, it is necessary to hold a certain amount of a tilt margin between the optical recording medium and the objective surface of the focusing lens.
Accordingly, the assignee of the present application has previously proposed a solid immersion lens, disclosed in U.S. patent application Ser. No. 11/073,608, in which a convex portion, for example, a convex portion such as a circular cone-like convex portion and a pyramid-like convex portion is formed on the objective side of the solid immersion lens and its tip end portion is processed as a planar portion to provide an objective surface so that, even when a distance between the objective surface and the optical recording medium is selected to be about several 10 s of nanometers, a tilt margin of approximately ±0.1 degrees can be maintained. Thus, it is possible to provide an optical pickup apparatus and an optical recording and reproducing apparatus in which recording and reproducing characteristics can be stabilized.
In the solid immersion lens having the convex portion formed on the objective side as described above, as the numerical aperture NA of the solid immersion lens is increased, that is, incidence angle of laser light is increased, a processed angle of the convex portion should be decreased so as not to disturb the incident light path of laser light. For example, when the solid immersion lens is shaped like the circular cone solid immersion lens, an angle of apex angle of the cone should be increased.
As a result, a bonding area of the solid immersion lens with a lens holding member for holding a solid immersion lens with a large numerical aperture NA is decreased, and there is a risk that the lens holding member will be detached from the solid immersion lens with application of a very small shock.
Similarly, the bonding area of the solid immersion lens with the lens holding member for holding this solid immersion lens is decreased as the radius of the spherical portion of the solid immersion lens is decreased or as a refractive index of the solid immersion lens is increased. Further, the above-mentioned bonding area is similarly decreased as the radius of the light-concentrating flat surface portion (that is, objective surface) at the tip end of the convex portion is increased.
In particular, in order to meet with the requirements in which the optical recording medium will be made high in density and in which the optical pickup apparatus will be made compact in size and will be made light in weight in the future, the numerical aperture of the solid immersion lens should be increased and the solid immersion lens should be microminiaturized. Therefore, even when the bonding area of the solid immersion lens with the lens holding member for holding the solid immersion lens can be decreased so much, the solid immersion lens should be stably fixed and held.